DESCRIPTION (Applicant's abstract): Aging is associated with an increased incidence of orthostatic hypotension in humans. Orthostatic hypotension has been shown to be associated with increased mortality. One mechanism that may be responsible for the age-related increases in orthostatic hypotension is attenuation of the vestibulo-sympathetic reflex. This reflex is a neural feedback system that regulates sympathetic neural outflow via afferent input from the vestibular otolith organs. Currently no data exist determining the influence of age on this reflex. Accordingly, the Primary aim of the present proposal is to test two hypotheses: 1) muscle sympathetic nerve activity (MSNA) responses to otolith organ stimulation are attenuated with aging; and 2) modulation of MSNA during the interaction between the vestibulo-sympathetic reflex and other cardiovascular reflexes is altered with aging. To address these aims, MSNA will be measured (microneurography) in a group of young and older subjects before and after otolith organ stimulation (head down neck flexion). Additionally, the vestibulo-sympathetic reflex responses will be measured during cardiovascular reflex activation (muscle afferents, baroreflex activation, and orthostatic challenge). The results of these studies will provide new and clinically important information concerning the function and importance of the vestibulo-sympathetic reflex in aging adult humans.